Wireless ad-hoc networks are decentralized wireless networks formed when a number of wireless communication devices, often referred to as nodes, decide to join together to form a network. Since nodes in wireless ad-hoc networks can operate as both hosts and routers, the network is easily reconfigured to meet existing traffic demands in a more efficient fashion than centrally managed wireless access networks. Moreover, wireless ad-hoc networks do not require the infrastructure required by these conventional access networks, making wireless ad-hoc networks an attractive alternative.
FIG. 1 illustrates an example wireless ad-hoc network 100 formed by a plurality of nodes 120 labeled A-H. As shown, each node 120 has an associated transmission range and is able to directly communicate with one or more other nodes 120 in the wireless ad-hoc network 100. Each node 120 may be stationary or in motion, such as a terminal that is being carried by a user on foot or in a vehicle, aircraft, ship, etc., and may be one of variety of communications devices including cellular, wireless or landline phones, personal data assistants (PDA), laptops, external or internal modems, PC cards, and any other similar device. The wireless ad-hoc network 100 may operate by itself, or may also be connected with an outside network 130 (e.g., the Internet) via one or more nodes (e.g., nodes A and C in FIG. 1).
Ultra-Wideband (UWB) is an example of communications technology (e.g., Multiband OFDM based UWB, ECMA-368, Impulse UWB, etc.) that may be implemented with wireless ad-hoc networks. UWB provides high speed communications over an extremely wide bandwidth. At the same time, UWB signals are typically transmitted in very short pulses that consume very little power. The output power of the UWB signal can be made low enough to look like noise to other RF technologies, making it less interfering.
A major challenge in wireless ad-hoc networks, including UWB networks, is the increased occurrence of hidden and/or exposed nodes. Hidden nodes in a wireless network refer to nodes that are out of range of other nodes or a collection of nodes. In the wireless ad-hoc network 100 of FIG. 1, node G at the far edge of the network 100 may be able to see node F in the middle of the network 100, but may not be able to see node C on the other end of the network 100. Accordingly, problems may arise when nodes G and C start to send packets simultaneously to node F. Since nodes G and C cannot sense each other's carriers, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is not sufficient to prevent collisions from occurring and data from being scrambled. Exposed nodes occur when a node is prevented from sending packets to other nodes due to a neighboring transmitter. In the wireless ad-hoc network 100 of FIG. 1, nodes A and G on the fringes of the network 100 may be out of range of one another while nodes E and H in the middle may be in range of each other. Here, if a transmission from node E to node A is taking place, node H is prevented from transmitting to node G since it concludes after carrier sense that it will interfere with the transmission by its neighbor node E. However, node G could still theoretically receive the transmission of node H without interference because it is out of range from node E. Thus, hidden and exposed nodes often cause problems for Media Access Control (MAC) because specific portions of the network are prevented from reusing the same bandwidth. A network's ability to reuse the same bandwidth over a given area is typically referred to as “spatial reuse.”
One example UWB communication system, ECMA-368, addresses the hidden node problem with a Distributed Reservation Protocol (DRP) where synchronizing beacon signals are broadcast by neighboring devices. The ECMA-368 standard, titled “High Rate Ultra Wideband PHY and MAC Standard,” second edition, December 2007, defines the PHY and MAC layers for a UWB network formed pursuant thereto and is incorporated herein by reference. A device in such a communication system that wishes to transmit information on one or more Media Access Slots (MAS) of a superframe can request to reserve (in advance) one or more MAS time slots using the DRP mechanism. Reservation negotiation is initiated by the device that will initiate frame transactions in the reservation, which is then referred to as the reservation “owner.” The device that will receive information is referred to as the reservation “target.” A typical beacon frame includes a DRP Information Element (IE) that identifies the MAS time slot reservations for that node (either as reservation owner or reservation target) with its neighboring nodes, as well as, among other elements, a DRP Availability IE that indicates a device's availability for new DRP reservations. The DRP IE may also be referred to as a DRP Reservation IE to distinguish it from the DRP Availability IE.
In this way, the DRP mechanism enlarges a “blocking area” around a communication link to guard against hidden nodes. However, the inventors have recognized that the DRP mechanism actually aggravates the exposed node problem because opportunities for concurrent transmissions are wasted. As discussed above, for a MAC to support simultaneous transmissions, both the neighboring nodes need to be either receivers or transmitters; simultaneous transmission is not possible when a receiver and transmitter are neighbors. Even for the case when both the neighboring nodes are transmitters or receivers, though, there is the problem of reverse direction traffic in the form of acknowledgments that greatly reduce spatial reuse.